1. Field of the Invention
The present invention relates to a communication method, apparatus, and system which feed back channel estimation information to a counterpart apparatus and, more particularly, to exchange of channel estimation information associated with MIMO (Multi Input Multi Output).
2. Description of the Related Art
A MIMO technique of transmitting frames by using a plurality of antennas has recently received a great deal of attention. A wireless communication system using the MIMO technique can realize high speed transmission without extending frequency bandwidth. MIMO is likely to be employed also in a wireless LAN standard IEEE802.11n which has currently been standardized. Particularly in the LOS (Line Of Sight) propagation environment, the channel state of each path may not be estimated even by a signal process on the receiver in a wireless communication system using MIMO. In this case, a data frame that is transmitted by using a plurality of antennas may not be able to be decoded on the receiver. One of the causes is that the channel state from the transmitter to the receiver is not obtained on the transmitter. If the transmitter does not know the condition of the received data on the receiver, the transmitter may transmit a frame using antennas by which the receiver may not correctly decode information. As a result, the band is wasted, which inhibits another communication.
To solve this problem, the transmitter may specify an antenna configuration used for transmission according to the channel state, and then transmits data so as to allow the receiver to decode information correctly.
For example, there is known a technique of feeding back information on a channel state estimated on the receiver to the transmitter and selecting a transmission method by the transmitter according to the channel state. This technique includes a method of switching a modulation scheme on the transmitter on the basis of channel information fed back from the receiver (see Jpn. Pat. Appln. KOKAI Publication No. 2002-290246), and a method of controlling the coding ratio (see Jpn. Pat. Appln. KOKAI Publication No. 2003-69531).
Feedback of channel estimation information for proper control of transmission using MIMO on the transmitter requires a technique unlike conventional communication using one antenna. For example, when a conventional technique is simply applied, frames are sequentially transmitted from a plurality of antennas one by one on the transmitter. The receiver measures the channel state for each antenna, and feeds it back to the transmitter. However, according to this method, the channel resource is greatly wasted depending on the number of antennas, and this feedback method is too complicated.